DE2450261A1 - METHOD FOR MANUFACTURING GRID ELECTRODES FOR ELECTRON TUBES - Google Patents

Description

PHILIPS PATMTVERWALTUITG GMBH, 2 Hamburg 1, Steindamm 94

Process for the production of grid electrodes for electron tubes

The invention relates to a method for producing grid electrodes for electron tubes, in which pyrolytic graphite by thermal decomposition of carbonaceous Gases deposited on a mandrel and the shaped body formed in this way separated from the mandrel and with lattice-shaped openings is provided.

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Graphite as a material in construction is more gas-filled and highly evacuated Tubes and electron tubes have already been widely used. Decisive for the application on this special sector are preferably the high-temperature properties, such as high sublimation or melting point (approx. 4200 K at 105 bar), relatively low vapor pressure, high thermal shock resistance and good thermal conductivity at high temperatures. The mechanical values are for operation in tubes (e.g. rectifier tubes and electron tubes) the strength, which increases with increasing temperature, is particularly important in the area around around 2500 K to around twice its value at 300 K. The electronic values are relatively high To emphasize the exit potential of about 4 Y, i.e. the thermal lattice emission, but also the secondary electron emission, are low.

Also known are grid electrodes made of graphite with pyrolytic Carbon layers are covered, as well as metal grid electrodes coated with pyrocarbon. From the DT-AS 1 194 988 grid electrodes are known which consist of pure pyrolytic graphite and which are opposite to the normal Graphite grid electrodes have the particular advantages associated with the anisotropic structure of pyrolytic graphite are. The comparatively much better mechanical stability and the associated ' dene greater operational reliability, "the possibility of significantly To produce finer-meshed grids, the better heat dissipation in the grid base due to the very good thermal conductivity parallel to the layering of the pyrolytic graphite and the better vacuum suitability due to the lack of Pores (well-oriented pyrolytic graphite has almost the theoretical density of 2.26 according to the ideal graphite single crystal).

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It is known from US Pat. No. 3,462,522 that the surface properties of the cathedral on which the pyxolytic graphite is precipitated, the structure of the formed! shaped body is determined. To improve the surface quality According to the US-PS, an electrographite mandrel is first treated with oxygen and then with a thin coating made of pyrolytic graphite - a relatively complex and expensive process.

From GB-PS 956 342 it is known several plates made of pyrolytic To connect graphite together that in the spaces between the plates pyrolytic graphite is deposited. On the other hand, from J. Materials Sei. 3 (1968) 559-561 shows that the coating of pyrolytic Graphite with pyrolytic graphite is not without problems and e.g. fails on (crystallographic) c-surfaces.

In no case can the previously cited publications that the production of pyrolytic graphite according to that described in DT-AS 1,667,649 and DT-PS 1,667,650 The hot gas pyrolysis procedure has been carried out.

The invention is based on the object of a method for producing grid electrodes for electron tubes of the initially introduced to create the type mentioned, by means of the grid electrodes with improved mechanical and electrical properties achieved and at the same time reduced the reject rates in production will.

This object is achieved according to the invention in that the molded body after the formation of the grid again with a a thin layer of pyrolytic graphite is coated and that the deposition on the mandrel and the repeated coating of the porous body provided with lattice-shaped openings so

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be made that the temperature of the carbon-containing gas is higher, preferably at least 100 K higher than is the temperature of the "horn" or of the shaped body.

In the investigations that led to the invention, it was found that the procedure according to DT-AS 1 667 649 and DT-PS 1 667 650 in the production of relatively thin-walled Hollow bodies made of pyrolytic graphite, as they are essentially the subject of the invention, are particularly well suited is. This particular suitability arises from the fact that the hot gas pyrolysis occurs in the entire deposition system occurring temperature differences a maximum of about 100 K. In addition, with hot gas pyrolysis there is inevitably a delayed cooling from the deposition temperature to room temperature, due to the thermal inertia of the separation system. Both of these factors have an effect extremely beneficial to the reduction of internal stresses in the separated pyrographite body.

For the production of hollow bodies (caps) from pyrolytic graphite, as they are required for different types of electron tubes, the following procedure was used: Normal synthetic fine-grained graphite was used as a substrate for the objects mentioned by turning, grinding and polishing into a shape corresponding to the cap brought. The pyrolytic graphite was deposited _{from propane (CH 8} ) at 2300 K and a pressure of about 2 mbar. With these parameters, the deposition rate was approximately 0.5 to 1.0 μm / min, ie for a cap with a wall thickness of 100 μm, the coating time was approximately 2 to 3 hours. This slow deposition rate is advantageous if the deposition itself is to be as homogeneous as possible, ie free from larger carbon particles (clusters). A perfect, ie as homogeneous as possible, structure of the pyrolytic graphite layer is of decisive importance for the subsequent perfect functioning of the grid electrode.

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In order to produce the required electrode shape, namely a grid, from the pyrographite caps, the former must be replaced by suitable Processing processes are further processed into a latticework. In general, processes such as electrical discharge machining are used for this, Electron beam cutting or the so-called "Electro Chemical Machining (ECM)" can be considered. Preferred is cutting using laser beams. Decisive for the selection of the suitable cutting or processing method is the "fineness" of the desired grid. The wall thickness of the cap is also an important parameter. One must assume that depending on the circumstances, different processes, including of course - possibly pure with coarse-meshed latticework mechanical processes (cutting, grinding) are to be used.

With COp lasers and solid-state lasers, lattices were made from pyrolytic Graphite is produced, the cylindrical part of which is provided with approximately 2 × 180 "windows" approximately 200 μm wide and 0.7 cm high had been. The web width between every 2 windows is about 100 μm. This results in a wall thickness of 100 μm

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a web cross-section of 10 / um = 10 ^ cm.

As images made in the SEM (scanning electron microscope) show, when the caps are cut, the layers of the cut into highly oriented pyrolytic graphite. This creates surfaces that are unfavorable in terms of vacuum technology and mechanics created. The former with regard to the increased tendency to degas, but also possibly increased gettering capacity for foreign gases -, the latter is to be understood to the effect that once the risk of small graphite particles breaking out increases and pressure loads due to temperature changes during operation lead to delamination of the layered pyrographite being able to lead. Electrically, there is an unfavorable side effect from electron-emitting edges and / or

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Peaks and thus a reduced high-voltage strength.

The inventive method step, the made according to the manufacturing process described above, made of pyrolytic graphite after the cutting process to form the mesh lattice to subsequently coat again with a thin, preferably 1 to 100 / um thick layer of pyrolytic graphite, the effect is that the This improves the mechanical and electrical properties as well as the vacuum properties. For example, by "sealing" the interstitial spaces with a second layer of practically impermeable pyrolytic graphite, delamination is inevitably blocked With the heating to temperatures in the range of about 2300 K in a high vacuum any foreign substances (O ₂ , Np, impurities) introduced (during cutting, eg in air) are expelled (evaporated) or rendered ineffective This can be seen in the fact that, with increasing layer thickness, the grid mesh size or the grid gaps can be narrowed to minimal distances, which is of great importance for use in the UHF range.

It is known that in addition to the parameters important for the homogeneity of a pyrolytic carbon ^{layer, substrate temperature, type of pyrolysis gas (e.g.} _{CH., C 2} ¹¹ O ' ^C "5 ^H 8 UrLd ^ ■ omo ^ - ° S ^e t CpH-, C-zHg and Homologues, CpHp and homologues, CgHg and others aromatics) and pyrolysis gas pressure determine the nature of the substrate, especially its surface, very significantly the homogeneity and degree of orientation of the deposited layer nucleated pyrolytic graphite and "continuously nucleated" pyrolytic graphite have been described. ER Stover, G&C Report No. 62-RL-2991M, has one

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detailed summary of the phenomenology of pyrolytic vision Graphite.

According to the invention, the homogeneity in the sense of suppression a so-called secondary anisotropy can be improved by the fact that the substrate, consisting of electrοgraphite, refractory metal such as molybdenum, tungsten or tantalum, metal carbides such as WC, MoC, TaC or FbC, preferably but made of vitreous carbon, polished as well as possible. It has been shown that such polished substrates very easy to remove from the mold (i.e. the pyrographite cap can be easily removed after cooling) and the entire surface of the caps, both inside and outside, becomes extremely smooth to high-gloss. Particularly The latter property is of particular importance for the high-voltage dielectric strength of a tube (no peak discharges).

The question of conductivity becomes purely for the operation of high-frequency tubes The electrode materials become more critical with increasing frequency, since the mechanism of current conduction changes plays more and more in the surface area of the respective conductor (skin effect). In the context of the method according to the invention it is advisable to reduce the anisotropy of so-called "as deposited" pyrolytic graphite by means of a high-temperature post-treatment at 3000 to 3800 K, especially at 3500 K, can still be improved significantly. This thermal post-treatment can be carried out under the pressure of an inert gas of 10 to 500 bar, in particular 100 bar.

Such post-treatment is handled in such a way that, for example, the deposited pyrolytic graphite in an inert gas (preferably noble gases) of 300 bar about one. Heated to 3300 K for an hour, then the pressure on reduced by about 10 bar and the temperature increased to 3800 to 3900 K for a further 10 minutes.

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The thermal or thermal-mechanical aftertreatment according to the invention enables conductivity values to be achieved which are of the order of magnitude of those of graphite intercalation compounds with about 2 to 3. 10 Λ ~ "cm" lie. This brings you into the range of good metallic conductivity (conductivity of copper about 5.8. 10 ⁵ XX ~ ¹ cm "* ¹ ).

The essential characteristics of the method according to the invention and its expedient configurations are summarized as follows:

a) Production of a hollow body (lattice cap) from pyrolytic graphite using the hot gas pyrolysis method.

b) Post-coating of the finished grid with a second layer of highly oriented pyrolytic graphite by means of hot gas pyrolysis.

c) Preferably use of the smoothest possible (polished) substrate cores. It is in particular use glassy carbon because it gives the best results. Investigated also became substrates made of polished tantalum and tungsten. These also lead to good results, but must be repolished before each reuse.

d) Preferably after-treatment of the electrode caps at high temperatures (3000 to 3800 K) alone or combined with the simultaneous application of high pressures, for example in a high pressure autoclave in inert gas under isostatic pressure conditions;

ρ pressures on the order of 10 bar are preferred.

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e) As a cutting method for the further processing of the grid cap to the grid electrode is preferred cutting by means of laser beams.

Patent claims:

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Claims (8)

245026] Patent claims: 1 .J Process for the production of grid electrodes for electron tubes, in which pyrolytic graphite is deposited on a mandrel by thermal decomposition of carbon-containing gases and the molded body thus formed is separated from the mandrel and provided with grid-shaped openings, characterized in that the molded body after the formation of the Grid is again coated with a thin layer of pyrolytic graphite and that the deposition on the mandrel and the repeated coating of the shaped body provided with lattice-shaped openings are carried out so that the temperature of the carbon-containing gas is higher than the temperature of the mandrel or the shaped body. 2. The method according to claim 1, characterized in that the temperature of the carbon-containing gas is at least 100 K higher than the temperature of the mandrel or the molded body. 3. The method according to claim 1 or 2, characterized in that the mandrel prior to the deposition of the pyrolytic Graphite is polished. 4. The method according to one or more of claims to 3 »characterized in that a mandrel made of vitreous Carbon is used. 5. The method according to one or more of claims to 4, characterized in that the shaped body after forming of the grid is covered with a 1 to 100 / µm thick layer of pyrolytic graphite. - 11 - 609819/0450 6. The method according to one or more of the claims 1 "to 5, characterized in that the grid electrodes are subjected to a thermal aftertreatment at 3000 to 3800 K. will. 7. The method according to claim 6, characterized in that the thermal aftertreatment under the pressure of an inert gas from 10 to 500 bar. 8. The method according to one or more of claims 1 to 7, characterized in that the shaped body by means of Laser is provided with grid-shaped openings. 609819 / 0AbU